Image formation in dry silver emulsions is based on the fact that the latent image generated by exposing the silver halide catalyzes the reaction of the insensitive silver salt with the reducing agent on heat treatment. However, as the reaction participant is not removed from the emulsion after development, progressive silver formation darkens the unexposed areas on extended storage, particularly in daylight. Image contrast decreases, and the image gradually disappears.
Numerous dry silver materials are known in which the developed image is supposed to be stabilized by adding to the light-sensitive emulsion certain compounds known as stabilizers. A number of pertinent publications are cited in Research Disclosure No. 17029, Section VIII (June 1978) and No. 29963, Section XIX (March 1989).
Known stabilizers can be classified almost entirely in the following groups of compounds:
Salts of mercury and other heavy metals PA0 Halogen compounds PA0 Heterocycles, substituted mostly with mercapto groups PA0 Acyclic sulfur compounds PA0 Acids and their derivatives PA0 Compounds that release bases
Various disadvantages accompany the use of these stabilizers. Salts of heavy metals and polyhalogenated compounds are no longer considered for consumer materials because of environmental damage. Sulfur compounds often impair sensitivity and maximum density or produce an opaque or yellow-tinted image background. Acids and heterocyclic compounds similarly impair maximum density; also, the latter can often be made only by complex syntheses and are therefore expensive. Many known stabilizers form noxious and unhealthy vapors during the heat treatment.
The problem in the invention involves making a dry silver material that can be manufactured easily and inexpensively with good stability in the developed image and the use of which does not generate an environmental or health risk.